Automotive vehicles may comprise passenger vehicles, trucks, vans, cross-over vehicles and other body variations. The bodies are constructed of load bearing structural members, floor members, closure members and the like. Such body members have been formed of cold rolled steel and galvanized steel and, in more recent years, from aluminum alloys. The respective body members are joined by welding, hemming, clinching, bolting, and like joining practices to form a body structure that is then ready for painting. Such an unpainted vehicle body structure is referred to as a “body-in-white” (sometimes referred to as BIW) because of the appearance of the bare metal elements of the body structure. Such vehicle bodies are then processed through long and sophisticated automotive phosphating and paint lines.
As suggested above, many vehicle bodies-in-white now contain portions that are formed from steel, galvanized steel and various aluminum alloys. A body comprising each of such ferrous, zinc, and aluminum materials is thoroughly cleaned and provided with a phosphate-containing surface conversion coating by immersion in an aqueous bath of phosphating composition. The phosphate conversion coatings chemically formed on the ferrous surfaces include iron (and sometimes zinc) and the phosphate conversion coatings on the aluminum surfaces comprise aluminum, and they are formed as a barrier layer on each exposed surface to provide corrosion resistance. These phosphate-containing conversion coatings have irregular surfaces that provide a tie-in base for a subsequently applied electrocoat paint layer. After phosphating, the vehicle bodies usually receive at least four paint layers to provide additional corrosion protection and color finishes. These paint layers include, in order of application: an electrocoat, a surface primer base coat, a base color coat, and a clear coat.
Now it is desired to make closure panels and other body members using magnesium alloys because of their favorable strength-to-weight ratio and because they can be formed as such body members and attached to complementary body members of magnesium, aluminum, or ferrous-based materials. However, magnesium is very reactive in aqueous solution and subject to galvanic corrosion, especially when coupled with steel alloys or aluminum alloys. When a magnesium body surface is immersed in an aqueous phosphating bath, magnesium dissolves in the bath, contaminates it, and adversely affects the quality of phosphate coating formed on nearby steel or aluminum surfaces.
It is an object of this invention to provide practices for forming conversion coatings and electrocoatings on bodies-in-white that comprise magnesium surfaces and aluminum alloy surfaces and/or steel surfaces, including galvanized steel surfaces.